Due to the natural resources being gradually depleted and the greenhouse effect becoming more and more serious, the electricity cost increases. Under this condition, various manufacturers have introduced various products that comply with environmental standards and energy conservation. Today, it is more stringent for a product to meet the requirements of environmental protection and green energy. In recent years, with the rapid development of LED-related technologies and the cost reduction, LEDs have been widely used in a variety of fields. In addition, an LED light has various advantages such as energy saving, excellent color saturation, long life, and environmental protection. Under today's environmental protection and energy-saving awareness, LED lights are gradually replacing conventional incandescent and fluorescent lamps, becoming the mainstream of the lighting products and expanding their applications in other fields.
For incandescent lamp, since the electro-optical conversion efficiency is not good, they do not meet the requirements of environmental protection and green energy. Generally, the luminous intensity of an incandescent lamp is adjusted through a silicon control dimmer, which however may produce a series of surge current to cause LEDs to flicker heavily, and furthermore, when the current flowing through the LEDs is too small, current harmonic distortion or even phase mismatch may occur. Thus, it is improper to simply use of a silicon control dimmer to adjust the luminous intensity of an LED light source.
Currently, the more mature technology is using PWM (pulse width modulation), which allows a transistor to be switched on/off at high frequency to drive LEDs to be bright and dark alternately at high frequency. The bright-dark frequency is much higher than the human eye can perceive. The brightness perceived by the human eye is based on the visual persistence, which is an average of the alternate brightness and darkness. By changing the on/off ratio in a period, a person can perceive a change of the luminous intensity of the LEDs. However, in addition to a high cost, the PWM control method can generate high-frequency EMI (electromagnetic interference). Furthermore, to keep away the human eyes from perceiving the flickering condition, the on/off period of the LEDs is relatively short. Therefore, when an LED light source is intended to operate at very low luminous intensity (for example 1% of the rated intensity), the conduction period is very short compared to a total period; that is, a circuit has to be turned on/off in very short time. However, an LED circuit usually takes some response time to change from an off state to a conduction state, thus causing the luminous intensity of the LED light source unable to meet the requirement.
If an LED light source is regulated by a constant current regulator (CCR), due to unavoidable drift of the output voltage of the power supply, the luminous intensity of the LED light source is unstable. Furthermore, a drift of the voltage may cause a drift of the wavelength, thus changing the color temperature. It is not easy for an LED light source controlled by a CCR to emit light less than 10% of its maximum luminous intensity.
The problem of luminous intensity drift and wavelength drift would become serious in some applications. For example, in a criminal investigation, a reagent such as luminol is usually sprayed at criminal scenes so as to determine blood stains, wherein ultraviolet (365 nm) or violet (405 nm) is used to excite the reagent to release a blue glow or fluorescence, so that a fluorescent-reaction result serving as evidence for analysis can be obtained. Due to the result depending on the wavelength and intensity of the excitation light, how to ensure these properties to remain unchanged and maintain accurate values in tests for evidence becomes a decisive factor on the credibility of the evidence.
In biotechnology identification, analysis has been developed from qualitative aspects to quantitative aspects. For example, in an experiment of gene transfer, in order to determine whether a specific gene is successfully transferred, an excitation light can be used to excite a fluorescent protein, which is often produced in a gene transfer, to confirm the presence of the fluorescent protein and the intensity of the fluorescent reaction. If there is a change of the wavelength of the excitation light, the fluorescent protein reaction may suddenly decrease to cause misjudgment. In some applications, such as determining the blood sugar concentration of diabetic people or the concentration of dioxin in food or articles, slight misjudgment is not allowed.
Data reproducibility is important for a criminal investigation, Dioxin concentration test, or blood glucose concentration test. For a commodity to be tested whether it contains dioxin, it is not allowed that the light source for the test has a drift in its light intensity or wavelength, because the drift may cause different results on different test dates, thus causing misjudgement and loss of data reproducibility. In a research for comparisons between experimental and control groups, a drift of the light intensity or wavelength may cause difficulty in analyzing the groups.
Furthermore, the human eye cannot detect the PWM signal, which takes only a few microseconds in each period. According to current technology, for example, a SONY camera can take up to thousands of pictures per second; a more advanced high-speed camera can take 50,000 pictures per second. Since the flickering of an LED light source can be recorded by a high-speed camera, the PWM control method is inapplicable to a test that requires a camera to take pictures.
Furthermore, patients lack of sunlight for a long time or people in high latitudes are susceptible to depression. Also, people lack of appropriate violet/UV light stimulation may lead to vitamin D deficiency. The aforementioned problems can be alleviated by providing appropriate blue light, violet light, or ultraviolet light to those persons. However, short-wavelength light carries higher energy each photon. A person may suffer skin cancer if exposed under short-wavelength light for a period of time. Nevertheless, it is quite helpful for vitamin D produced in the human body if a person is exposed under UV light with a specific wavelength and an extremely narrow bandwidth. In other words, the benefits would be significantly reduced if a person is exposed under UV light of other wavelengths.
In view of the foregoing, it is important for the voltage applied to an LED light source to be controlled at a precise level, so that the wavelength and luminous intensity of the LED light source can be controlled at a constant level. Conventional technology, such as PWM or CCR, has disadvantages, wherein the PWM method incurs higher cost and may cause high frequency EMI; the CCR method may cause a change of the luminous intensity or the color temperature. Particularly, either the CCR or PWM method is unsuitable for a light working at a low brightness state. The present invention provides an adjustable power supply, which can supply precise constant current to avoid a drift of its output voltage, so that a DC load can maintain its stable operation, and thus a light source using the power supply can ensure its quality.